Optical element and light guide element

Abstract

An optical element comprising: a cholesteric liquid crystal layer obtained by cholesteric alignment of a liquid crystal compound, in which the cholesteric liquid crystal layer has a liquid crystal alignment pattern in which a direction of an optical axis derived from the liquid crystal compound changes while continuously rotating in at least one in-plane direction, a helical pitch of a helical axis direction in the cholesteric alignment gradually changes in a thickness direction of the cholesteric liquid crystal layer, and the cholesteric liquid crystal layer has a peak of reflection at each of a first wavelength λ and a second wavelength λ/2.

What is claimed is: 
     
         1 . An optical element comprising:
 a cholesteric liquid crystal layer obtained by cholesteric alignment of a liquid crystal compound,   wherein the cholesteric liquid crystal layer has a liquid crystal alignment pattern in which a direction of an optical axis derived from the liquid crystal compound changes while continuously rotating in at least one in-plane direction,   a helical pitch of a helical axis direction in the cholesteric alignment gradually changes in a thickness direction of the cholesteric liquid crystal layer, and   the cholesteric liquid crystal layer has a peak of reflection at each of a first wavelength λ and a second wavelength λ/2.   
     
     
         2 . The optical element according to  claim 1 ,
 wherein the cholesteric liquid crystal layer has regions where diffraction efficiencies of light having the second wavelength λ/2 are different in a plane.   
     
     
         3 . A light guide element comprising:
 the optical element according to  claim 1 ; and   a light guide plate.   
     
     
         4 . The light guide element according to  claim 3 ,
 wherein the optical element is an incidence element that causes light having the first wavelength λ and light having the second wavelength λ/2 to be incident to the light guide plate at an angle where the light is totally reflected.   
     
     
         5 . The light guide element according to  claim 3 , further comprising:
 an incidence element that causes light to be incident to the light guide plate; and   an emission element that emits light from the light guide plate,   wherein the optical element is an emission element that causes light having the second wavelength λ/2 to be emitted from the light guide plate, and   the cholesteric liquid crystal layer has regions where diffraction efficiencies of the light having the second wavelength λ/2 are different in a plane.   
     
     
         6 . The light guide element according to  claim 5 ,
 wherein in the cholesteric liquid crystal layer, a diffraction efficiency of light having the second wavelength λ/2 gradually increases in a direction away from the incidence element.   
     
     
         7 . The light guide element according to  claim 5 ,
 wherein the regions where diffraction efficiencies of the light having the second wavelength λ/2 are different in a plane have the lowest diffraction efficiencies of the light having the second wavelength λ/2 in a region on an incident element side, and the highest diffraction efficiency of the light having the second wavelength λ/2 in a region on the opposite to the incident element side.